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random_seed: Rewrite the generic clock() based seed code

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The new code is faster and reuses the previous state in case of
multiple calls.

The previous code could easily end up in near-infinite loops,
if the difference between two clock() calls never was larger than
1.

This makes fate-parseutils finish in finite time when run in wine,
if CryptGenRandom isn't available (which e.g. isn't available if
targeting Windows RT/metro).

Patch originally by Michael Niedermayer but with some modifications
by Martin Storsjö.

Signed-off-by: Martin Storsjö <martin@martin.st>
This commit is contained in:
Michael Niedermayer 2012-06-11 11:47:59 +02:00 committed by Martin Storsjö
parent e61b8fa560
commit 01d245ef43
1 changed files with 30 additions and 21 deletions
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51
libavutil/random_seed.c
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@ -31,8 +31,11 @@
#include <math.h>
#include <time.h>
#include "internal.h"
#include "intreadwrite.h"
#include "mem.h"
#include "timer.h"
#include "random_seed.h"
#include "sha.h"
static int read_random(uint32_t *dst, const char *file)
{
@ -53,34 +56,40 @@ static int read_random(uint32_t *dst, const char *file)
static uint32_t get_generic_seed(void)
{
struct AVSHA *sha = av_sha_alloc();
clock_t last_t = 0;
int bits = 0;
uint64_t random = 0;
unsigned i;
float s = 0.000000000001;
static uint64_t i = 0;
static uint32_t buffer[512] = { 0 };
unsigned char digest[20];
uint64_t last_i = i;
for (i = 0; bits < 64; i++) {
for (;;) {
clock_t t = clock();
if (last_t && fabs(t - last_t) > s || t == (clock_t) -1) {
if (i < 10000 && s < (1 << 24)) {
s += s;
i = t = 0;
} else {
random = 2 * random + (i & 1);
bits++;
}
if (last_t == t) {
buffer[i & 511]++;
} else {
buffer[++i & 511] += (t - last_t) % 3294638521U;
if (last_i && i - last_i > 4 || i - last_i > 64)
break;
}
last_t = t;
}
#ifdef AV_READ_TIME
random ^= AV_READ_TIME();
#else
random ^= clock();
#endif
random += random >> 32;
return random;
if (!sha) {
uint32_t seed = 0;
int j;
// Unable to allocate an sha context, just xor the buffer together
// to create something hopefully unique.
for (j = 0; j < 512; j++)
seed ^= buffer[j];
return seed;
}
av_sha_init(sha, 160);
av_sha_update(sha, (const uint8_t *) buffer, sizeof(buffer));
av_sha_final(sha, digest);
av_free(sha);
return AV_RB32(digest) + AV_RB32(digest + 16);
}
uint32_t av_get_random_seed(void)